Cart-based visibility system

ABSTRACT

A cart has at least three wheels. It has one or more loop antennas, and a radio transceiver connected with the antennas. The antennas and transceiver operate at a frequency lower than 1 megahertz. The loop antennas are each at least 0.2 square meters in area. The cart is moved to an area such as a room, and the transceiver communicates with various RF tags in the room. Because of the antenna configuration, the portion of spectrum employed, and the power levels used, the cart is able to communicate with most if not all of the RF tags in the room. The cart can then be moved to another area such as another room, and the process repeated. In this way an inventory of tags can be made without expensive permanently installed infrastructure. The system is robust against interferers such as large metal objects and intervening objects.

This application claims priority from U.S. application Ser. No.60/805,972, filed Jun. 27, 2006, which application is herebyincorporated herein by reference for all purposes.

BACKGROUND

A corporation keeping track of assets (e.g., desks, chairs, computers)will often try to accomplish this by means of assets tags. Each assettag is adhesively secured to an asset, and bears a tracking number orbar code or both. Carrying out an inventory is a tedious process,requiring a lot of people and a lot of time. It is unworkable to carryout a building-wide inventory more often than about once a year.

Given the problems with numbered or bar-coded tags, many investigatorshave tried to keep track of assets using radio tags such as RFID tags.But locating radio tags in a large or multi-room space is not easy.Among the reasons why this is not easy are the following.

Range. Most radio tags work only if the reader is immediately adjacentto the tag. For example, many RFID tags can only be read at a distanceof inches. Among the RFID tags that can be read from more than a fewinches away, many can only be read from more than a few inches away inthe special case where a high-gain antenna is pointed directly at thetag.

Collisions and area reads. Most radio tags simply respond when powered.If many such tags are nearby to each other, all within a reading area ofa reader, what generally happens is that all the tags respondsimultaneously, and the collision often means that few or none of thetags can actually be read. Complicated collision-avoidance anddisambiguation schemes can be attempted but such schemes often do notwork well.

Detuning. Many radio tags many recent RFID tags, get detuned if they arenearby to large pieces of metal or other conductors. With many prior-artsystems, a detuned tag may be treated as a tag that does not exist, asit will fail to respond to queries.

Skin depth. Many radio tags, especially many recent RFID tags, simplycannot be reached if there are intervening objects blocking the RFsignals. The RF signals are unable to pass back and forth through theintervening objects.

Some approaches that have attempted to overcome these problems areprodigiously expensive.

It would thus be extremely desirable to have a reasonably priced systemthat would permit area reads, over a substantial range (many feet),robust against detuning, and effective even in the face of interveningobjects.

SUMMARY OF THE INVENTION

A cart has at least three wheels. It has one or more loop antennas, anda radio transceiver connected with the antennas. The antennas andtransceiver operate at a frequency lower than 1 megahertz. The loopantennas are each at least 0.2 square meters in area. The cart is movedto an area such as a room, and the transceiver communicates with variousRF tags in the room. Because of the antenna configuration, the portionof spectrum employed, and the power levels used, the cart is able tocommunicate with most if not all of the RF tags in the room. The cartcan then be moved to another area such as another room, and the processrepeated. In this way an inventory of tags can be made without expensivepermanently installed infrastructure. The system is robust againstinterferers such as large metal objects and intervening objects.

DESCRIPTION OF THE DRAWING

FIG. 1 shows how signal strength drops off with distance.

FIG. 2 shows the information of FIG. 1 but on a logarithmic scale.

FIG. 3 shows how signal strength drops off as a tag that is ten feetaway moves off axis.

FIG. 4 shows how signal strength drops off as a tag that is five feetaway moves off axis.

FIG. 5 shows how signal strength drops off as a tag that is five feetaway rotates about a Z axis.

FIG. 6 shows how signal strength drops off as a tag that is five feetaway rotates about a Z axis while positioned 45 degrees left of a vectornormal to the antenna.

FIG. 7 shows how signal strength changes as a tag that is five feet awayrotates about an axis normal to the antenna.

DETAILED DESCRIPTION

The radio tags employed can, for example, be tags such as thosedescribed in U.S. Pat. No. 7,049,963 entitled “Networked RF tag fortracking freight” and assigned to the same assignee as the presentinvention, which patent is incorporated herein by reference.

The transceiver on the cart can, for example, be a transceiver such asthat described in copending U.S. application number [tunable loop],filed ______ and assigned to the same assignee as the present invention,which application is incorporated herein by reference. The transceivercan transmit at, say, a predetermined multiple of 32768 Hertz (thestandard watch crystal frequency) such as 65 kHz or 133 kHz. It uses anantenna that a copper coil forming a rectangle 21 inches by 13 inches.The cart may carry three such antennas, each orthogonal to the other twoantennas. The transceiver can switch from one antenna to the next, andeven if one antenna is not well coupled with a particular tag in a room,very likely one of the other two antennas will turn out to be wellcoupled with that particular tag.

As described in more detail in the copending application, thetransceiver has an antenna tuner which is used in real time to achievean optimal impedance coupling between the transceiver and the antenna.In addition, in an exemplary embodiment, the transceiver is able to betuned upwards or downwards from a nominal frequency. By use of theup-tuning and down-tuning, and by use of the antenna tuner, and by useof the several loop antennas, it turns out to be possible to communicateeven with “difficult” tags which old prior-art transceivers might not beable to reach. For example if a tag is “detuned” by proximity to a largebody of metal, the transceiver described here will likely be able tocommunicate with the tag where old prior-art transceivers almostcertainly would not be able to do so.

Some examples based upon actual signal strength measurements illustratethat the system according to the invention works better than prior-artsystems. It will also be appreciated that this knowledge of the mannerin which signal strength drops off can permit a cart and transceiver andseveral antennas to localize a tag in 3-dimensional space about thecart.

FIG. 1 shows how signal strength drops off with distance. In thisexperiment, measurements were made of signal versus distance on thecenter line of the antenna with the tag axis pointed at the center ofthe antenna. This experimental result is important for two reasons.First, it shows that it is possible to read a tag even though it is 16feet away, and in this respect the result is very different from what isobtained with many earlier RFID technologies. Second, it may beappreciated that the received signal strength may be used as anindicator of the distance to the tag. FIG. 2 shows the information ofFIG. 1 but on a logarithmic scale. In this experiment and in the onesdescribed below, the antenna was 21 inches by 13 inches, wound on aframe made of polymethyl methacrylate. It turns out that signal strengthversus distance for this combination of tag and base station can be verywell described by the equation S=85000/(R^(2.5)) where R is in feet,with saturation effects occurring between 0 and 5 feet. This may beextended with additional antennas.

FIG. 3 shows the results of an experiment in which signal strengthmeasurements were made at the base station (transceiver and antennawhich could be on a cart) with the tag on a 10-foot radius arc in frontand to the left (CCW) of the antenna. The tag was oriented so that itwas at the same height as the antenna and the tag's axis was pointed atthe center of the antenna for all readings. This, too, is important fortwo distinct reasons. First, it shows that it is possible to read a tageven though the tag is far off (as much as 70 degrees) from the antennaaxis. In this respect the result is very different from what would beseen with many RFID technologies, where a tag that is ten feet away willsimply be unreadable at all even if it is only ten or twenty degrees offthe antenna axis. Second, it may be appreciated that the received signalstrength may be used as an indicator of the extent to which the tag isoff the antenna axis. Similarly, FIG. 4 shows how signal strength dropsoff as a tag that is five feet away moves off axis.

FIG. 5 shows how signal strength drops off as a tag that is five feetaway rotates about a Z axis. In this experiment, measurements of signalvs rotation of the tag about the Z axis were made with the tagpositioned 5 feet from the antenna on the center line and the tagantenna normal axis pointed at the center of the antenna. In this casethe tag is on the antenna axis, and the tag rotates about a Z axis,defined as an axis that is perpendicular to the vector normal to thetag. Stated differently, if one draws a line from the antenna to thetag, the Z axis is perpendicular to that line. This is important becauseit shows that it is possible to read a tag even though the tag is not“facing” the antenna. In contrast, with many RFID technologies, a tagmay be read only if it is facing the antenna (or facing directly awayfrom the antenna). Stated differently, the experimental result was thatrotation of the tag about its Z axis when the normal vector to itsantenna is in the horizontal plane causes a decrease in signal strengthread proportional to the cosine of the angle between the tag antennanormal and the field direction.

Similarly, FIG. 6 shows how signal strength drops off as a tag that isfive feet away rotates about a Z axis while positioned 45 degrees leftof a vector normal to the antenna. With the tag located at 5 feet fromthe antenna and off the antenna axis by 45 degrees CCW, the tag wasrotated about its Z axis by a full 360 degrees. The zero-degree pointwas with the tag antenna's axis pointed directly at the antenna'scenter. It shows that for many angles the tag can be read, despite beingoff the antenna normal vector and despite its not facing the antenna. Itappears that field lines when the tag is off the antenna's center lineare not radial, and in fact with the tag at the 45-degree point 5 feetfrom the tag, the field is offset 25 degrees from radial.

FIG. 7 shows how signal strength changes as a tag that is five feet awayrotates about an axis normal to the antenna. With the tag located at 5feet from the antenna on the antenna's center line, the tag was rotatedabout the normal vector to the tag's antenna. This shows that so long asthe tag faces the antenna, it does not matter if the tag is rotatedwithin the plane of the tag face. Stated differently, rotation of thetag about the normal vector to its antenna doesn't change the signalstrength read. If three antennas are employed, and if the antennas arenot all coplanar or parallel to each other, these results indicate thatto some extent the received signal strength on the three antennas willpermit localizing the tag in three-dimensional space. Preferably theantennas would each be orthogonal to the other two, but orthogonality isnot required. (Even if the antennas are not orthogonal, mathematicalcomputations or lookup tables would permit approximating 3-D locationsbased upon signal strengths.)

FIG. 8 shows an exemplary cart 81 assembly. It includes a cart 88, andwheels 82 a, 82 b, and 82 c. Superposed on the cart 88 is the cubeantenna 83. The cube antenna 83 includes square antennas 84, 85, 86, and87. Antennas 86 and 87 are parallel with the floor. It is not, ofcourse, required that the antennas be square, and indeed they could becircular in shape or other random shapes. Square antennas are, in someways, easier to fabricate and to assemble into three axes as portrayedhere.

FIG. 9 shows three cube antennas 96, 93, and 92, each connected with arespective router 95, 94, and 91. A camera 97 is nearby. The three cubeantennas, each with three orthogonal antennas, can detect and localize atag in five dimensions—3D position as well as tag pitch and yaw. Theexperimental results suggest a three-dimensional resolution of plus orminus two to six inches, and an angle resolution to within 10 to 20degrees depending upon the angle.

What follows is one example of a method according to the invention. Abuilding has at least first second, and third rooms. A cart is provided,the cart having at least three wheels, the cart having a first firstloop antenna and a radio transceiver, the first loop antennacommunicatively coupled with the radio transceiver, the radiotransceiver operating at a frequency lower than 1 megahertz, the firstfirst loop antenna having an area greater than 0.2 square meters. Thecart is placed in the first room. While the cart is in the first room,the cart communicates by means of the first loop antenna with at leastfirst, second, and third tags, each of the first, second, and third tagsbeing at least five feet from the cart, the communication with thefirst, second, and third tags achieved without reorienting the firstloop antenna, the communication with the first, second, and third tagseach yielding an identification of each of the first, second, and thirdtags. The cart is moved to the second room. While the cart is in thesecond room, the cart communicates by means of the first loop antennawith at least fourth, fifth, and sixth tags, each of the fourth, fifth,and sixth tags being at least five feet from the cart, the communicationwith the fourth, fifth, and sixth tags achieved without reorienting thefirst loop antenna, the communication with the fourth, fifth, and sixthtags each yielding an identification of each of the fourth, fifth, andsixth tags. The cart is moved to the third room. While the cart is inthe third room, the cart communicates by means of the first loop antennawith at least seventh, eighth, and ninth tags, each of the seventh,eighth, and ninth tags being at least five feet from the cart, thecommunication with the seventh, eighth, and ninth tags achieved withoutreorienting the first loop antenna, the communication with the seventh,eighth, and ninth tags each yielding an identification of each of theseventh, eighth, and ninth tags. The first, fourth, and seventh tags areeach fixed respectively to the first, second, and third rooms, wherebythe cart is able to know unequivocally which room it is in. The second,third, fifth, sixth, eighth, and ninth tags are each attached torespective movable objects. Optionally, the cart further comprises asecond loop antenna not coplanar with the first loop antenna, the secondloop antenna communicatively coupled with the radio transceiver, themethod further comprising the steps of selectively operatively couplingthe first loop antenna and not the second loop antenna to thetransceiver; and selectively operatively coupling the second loopantenna and not the first loop antenna to the transceiver.

One application of the system and method according to the invention iscorporate inventory control in a building. Instead of, or in additionto, applying traditional numbered or bar-coded inventory labels toassets, RF tags are applied to assets. A cart according to the inventionis pushed through the rooms of the building, one by one. The first timethrough the building is a time-consuming trip as the system must detectand disambiguate all or nearly all of the tags in the building, andassociate each tag with a location such as a particular room. Subsequenttrips through the building, however, may be faster, since the system canlook for a particular tag in a room where it was previously detected,addressing that tag individually and thus saving the step of detectingit by seeking out a tag that had not previously been detected in thatroom.

It will be appreciated that many tags working at higher frequencies(e.g. the tens of gigahertz used with some RFID tags) are able to beread very quickly, one after the other. In contrast, the systemdescribed here is only able to proceed at a bandwidth of a few hundredbits per second. This is not, however, a big problem because the cartcan be moved from one room to the next as slowly or as quickly as isneeded. In an exemplary embodiment, the cart will have a display showingprogress in a particular room, and will let a human operator know whenthe cart is finished in a room and can be moved to a different room.

In an exemplary embodiment, each room has a fixed tag that permits thecart to know which room it is in. The cart may then enumerate the roomsduring an initial trip through the building. On later trips through thebuilding, the cart can “check off” the rooms one by one and can alertthe human operator if some particular room is overlooked.

It will be appreciated that in many buildings, there is a cleaning crewwhich passes through the building periodically, such as daily or everyfew days. The cleaning cart pushed by the crew can carry the transceiverand antennas described here, and thus represents little or no additionallabor cost beyond the fixed labor cost of the cleaning crew.

In a hotel, the cart can be a housekeeping cart pushed by a housekeeper.The cart can, among other things, monitor that nothing has been stolen(e.g. a television or hair dryer or ironing board) as well as monitoringthe housekeeping status of each room (not yet cleaned, cleaned for areturning guest, and cleaned for a new check-in).

In a grocery store, a customer grocery cart may may carry thetransceiver and antenna described here. If so, the cart can detectfixed-position tags permitting the cart to learn where it is in thestore.

In all these cases an exemplary cart will also have an 802.1 lb/gwireless node in communication with 802.1 lb/g access points located atvarious positions within the building. By means of a WiFi link the cart(or carts, if there is more than one) can communicate with a centralhost.

In a building where high-value items are stored, this system will permitreal-time or near-real-time visibility of the items. One example is ahospital in which high-value items such as stents or artificial jointsare stored. Each stent or joint is tagged with a tag of the typedescribed here, and when a cart according to the invention passesthrough the room, the stents and joints may be counted and located. Anychanges from the previous inventory can be annunciated.

Those skilled in the art will have no difficulty devising myriad obviousvariants and improvements upon the invention, all of which are intendedto be encompassed by the claims which follow.

1. A method for use with a building comprising at least first second,and third rooms, the method comprising the steps of: providing a cart,the cart having at least three wheels, the cart having a first firstloop antenna and a radio transceiver, the first loop antennacommunicatively coupled with the radio transceiver, the radiotransceiver operating at a frequency lower than 1 megahertz, the firstfirst loop antenna having an area greater than 0.2 square meters;placing the cart in the first room; while the cart is in the first room,communicating by means of the first loop antenna with at least first,second, and third tags, each of the first, second, and third tags beingat least five feet from the cart, the communication with the first,second, and third tags achieved without reorienting the first loopantenna, the communication with the first, second, and third tags eachyielding an identification of each of the first, second, and third tags;moving the cart to the second room; while the cart is in the secondroom, communicating by means of the first loop antenna with at leastfourth, fifth, and sixth tags, each of the fourth, fifth, and sixth tagsbeing at least five feet from the cart, the communication with thefourth, fifth, and sixth tags achieved without reorienting the firstloop antenna, the communication with the fourth, fifth, and sixth tagseach yielding an identification of each of the fourth, fifth, and sixthtags; moving the cart to the third room; while the cart is in the thirdroom, communicating by means of the first loop antenna with at leastseventh, eighth, and ninth tags, each of the seventh, eighth, and ninthtags being at least five feet from the cart, the communication with theseventh, eighth, and ninth tags achieved without reorienting the
 1. Amethod for use with a building comprising at least first second, andthird rooms, the method comprising the steps of: providing a cart, thecart having at least three wheels, the cart having a first first loopantenna and a radio transceiver, the first loop antenna communicativelycoupled with the radio transceiver, the radio transceiver operating at afrequency lower than 1 megahertz, the first first loop antenna having anarea greater than 0.2 square meters; placing the cart in the first room;while the cart is in the first room, communicating by means of the firstloop antenna with at least first, second, and third tags, each of thefirst, second, and third tags being at least five feet from the cart,the communication with the first, second, and third tags achievedwithout reorienting the first loop antenna, the communication with thefirst, second, and third tags each yielding an identification of each ofthe first, second, and third tags; moving the cart to the second room;while the cart is in the second room, communicating by means of thefirst loop antenna with at least fourth, fifth, and sixth tags, each ofthe fourth, fifth, and sixth tags being at least five feet from thecart, the communication with the fourth, fifth, and sixth tags achievedwithout reorienting the first loop antenna, the communication with thefourth, fifth, and sixth tags each yielding an identification of each ofthe fourth, fifth, and sixth tags; moving the cart to the third room;while the cart is in the third room, communicating by means of the firstloop antenna with at least seventh, eighth, and ninth tags, each of theseventh, eighth, and ninth tags being at least five feet from the cart,the communication with the seventh, eighth, and ninth tags achievedwithout reorienting the first loop antenna, the communication with theseventh, eighth, and ninth tags each yielding an identification of eachof the seventh, eighth, and ninth tags.
 2. The method of claim 1 whereinthe first, fourth, and seventh tags are each fixed respectively to thefirst, second, and third rooms, and wherein the second, third, fifth,sixth, eighth, and ninth tags are each attached to respective movableobjects.
 3. The method of claim 1 wherein the cart further comprises asecond loop antenna not coplanar with the first loop antenna, the secondloop antenna communicatively coupled with the radio transceiver, themethod further comprising the steps of: selectively operatively couplingthe first loop antenna and not the second loop antenna to thetransceiver; and selectively operatively coupling the second loopantenna and not the first loop antenna to the transceiver.
 4. The methodof claim 1 further comprising the steps of: when the cart is in thefirst room, cleaning the first room; when the cart is in the secondroom, cleaning the second room; and when the cart is in the third room,cleaning the third room.
 5. A system for use in a building, the buildinghaving at least first, second, and third rooms, the system comprising: acart, the cart having at least three wheels, the cart having a firstfirst loop antenna and a radio transceiver, the first loop antennacommunicatively coupled with the radio transceiver, the radiotransceiver operating at a frequency lower than 1 megahertz, the firstfirst loop antenna having an area greater than 0.2 square meters; in thefirst room, at least first, second, and third radio tags, each disposedto receive signals from the cart at a frequency lower than 1 megahertz,each of the first, second, and third radio tags in radio communicationwith the cart when the cart is in the first room; in the second room, atleast fourth, fifth, and sixth radio tags, each disposed to receivesignals from the cart at a frequency lower than 1 megahertz, each of thefourth, fifth, and sixth radio tags in radio communication with the cartwhen the cart is in the second room; in the third room, at least sixth,seventh, and eighth radio tags, each disposed to receive signals fromthe cart at a frequency lower than 1 megahertz, each of the sixth,seventh, and eighth radio tags in radio communication with the cart whenthe cart is in the third room.
 6. The system of claim 5 wherein thefirst, fourth, and seventh tags are each fixed respectively to thefirst, second, and third rooms, and wherein the second, third, fifth,sixth, eighth, and ninth tags are each attached to respective movableobjects.
 7. The system of claim 5 wherein the cart further comprisescleaning supplies.
 8. The system of claim 5 wherein the cart furthercomprises a holder for a trash bag.